A leathery sheet-like material formed primarily of a fibrous base and polyurethane has excellent features which natural leather lacks. Thus this material is widely used in various applications. Particularly, a leathery sheet-like material using a polyester-based fibrous base excels in lightfastness. Therefore, the range of use of this sheet-like material has been extended year after year to cover applications including clothing, chair upholstery, and automobile interior materials.
In producing such a leathery sheet-like material, the combination of the following steps is generally adopted: a step where a fibrous base is impregnated with an organic solvent solution of polyurethane; and a step where subsequently, the fibrous base thus obtained is immersed in water or an aqueous organic solvent solution that is a non-solvent for polyurethane to achieve wet coagulation of polyurethane. Examples of said organic solvent for polyurethane include water-miscible solvents such as N,N-dimethyl formaldehyde. However, organic solvents generally are highly harmful to humans. Therefore, there is a strong demand for a technique that does not use an organic solvent during production of a sheet-like material.
By way of a specific means for solving this problem, studies have been performed for development of a method use water-dispersed polyurethane, which is produced by dispersing polyurethane in water, in place of conventional organic solvent type polyurethane. However, a sheet-like material which is formed by impregnating or providing a fibrous base with water-dispersed polyurethane has a problem in that the texture tends to be hard. The following two points may be mentioned as main reasons for this: polyurethane is unevenly distributed in a fibrous base due to the occurrence of migration, and polyurethane strongly grasps entangled portions of fibers in the fibrous base. Studies are being made aiming to solve these problems.
As for the restriction of migration, i.e., the former problem, a technique is proposed whereby an inorganic salt is added to water-dispersed polyurethane as a thermosensitive coagulant so that the fluidity of the water-dispersed polyurethane is reduced by heating (see Patent Document 1). However, Patent Document 1 discloses no technique to solve the latter problem caused by polyurethane strongly grasping entangled portions of fibers in a fibrous base. Therefore, it follows that the texture of the sheet-like material is strongly affected by the flexibility of the polyurethane itself. It is conceivable that flexible polyurethane with low crystallinity is used for the purpose of developing a flexible texture in the sheet-like material. However, in this case, wear resistance will deteriorate, and sandpaper etc. used for grinding and napping will be easily clogged, thereby making it difficult to obtain satisfactory napping quality.
On the other hand, as for the restriction of the grasping of entangled points in fibers, i.e. the latter problem, an arrangement is proposed in which an association type thickener is added to water-dispersed polyurethane so that the polyurethane in a fibrous base forms a porous structure (see Patent Document 2). In this proposal, as polyurethane becomes porous, the bonding area between fibers and polyurethane decreases, resulting in a decrease in the grasping force at entangled points in fibers. However, if an association type thickener is added to water-dispersed polyurethane, stickiness is caused by the association type thickener in the sheet-like material which is impregnated with polyurethane. Therefore, a cleaning step for the association type thickener is required, resulting in low productivity. Furthermore, since the association type thickener is in the dissolved state in water-dispersed polyurethane, pores in the polyurethane structure are formed in portions where the association type thickener is present, and therefore, it is impossible to increase pore diameters, failing to obtain a significant texture-softening effect for the sheet-like material.
Furthermore, to provide a technique for obtaining a porous polyurethane structure, there is a proposal of a coated sheet produced by applying a coating material prepared by adding a foam stabilizer to polyurethane followed by mechanical foaming (see Patent Document 3). In the case of this proposal, a fibrous base surface is coated with mechanically foamed water-dispersed polyurethane to form a polyurethane film having a porous structure. However, if a fibrous base is impregnated with foamed water-dispersed polyurethane, then the foam may disappear during impregnation, making it impossible to form a porous polyurethane structure in the fibrous base.
Furthermore, apart from the above, there is another proposal of full-grain-like artificial leather produced by adding thermally expandable capsules to water-dispersed polyurethane using it to coat a fibrous base (Patent Document 4). In the case of this proposal, the thermally expandable capsules are expanded in the polyurethane to form a porous structure, and the impregnation of a fibrous base with polyurethane serves to form a porous polyurethane structure inside the fibrous base. However, this is accompanied by coloring due to thermal burns caused by the thermally expandable capsules added, and hardening of the texture of the sheet-like material as a result of the hardness of the thermally expandable capsules themselves.
Thus, for production of a sheet-like material, there have been no successful efforts in producing a flexible sheet-like material in a process that uses no organic solvents.